(1) Field of the Invention
This invention relates to a hydrogen-absorbing alloy electrode used as a negative electrode of an alkaline storage cell.
(2) Description of the Prior Art
Conventional storage cells include nickel-cadmium or other alkaline cells and lead cells. Drawing current attention is a metal oxide-hydrogen alkaline storage cell comprising a hydrogen-absorbing alloy electrode, which can realize lightness, large capacity and high energy density. Used for such an electrode is a hydrogen-absorbing alloy such as LaNi.sub.5 (disclosed in U.S. Pat. No. 4,004,943), LaNi.sub.4 Co, or LaNi.sub.4.8 Fe.sub.0.2. LaNi.sub.4 Co and LaNi.sub.4.8 Fe.sub.0.2 have both been developed from LaNi.sub.5. Also having been developed for use in the above electrode are hydrogen-absorbing alloys including Misch metal (Mm), which is a mixture of rare earth elements such as La, Ce, Pr, Nd and Sm (Japanese Patent Publication Kokai No. 62-20245).
A negative electrode using a hydrogen-absorbing alloy reacts on a surface of the hydrogen-absorbing alloy and absorbs hydrogen generated while the cell is charged into the alloy. If the hydrogen-absorbing alloy used for the negative electrode absorbs an especially large amount of hydrogen and so is excellent in functioning as a catalyst, the negative electrode obtains a high energy density. Moreover, a cell having such an electrode is completely compatible with a nickel-cadmium cell having a cadmium negative electrode since both of the cells have the similar charge/discharge potential.
The cell employing the hydrogen-absorbing alloy as the negative electrode has an energy density which is approx, 1.5 times as high as that of a nickel-cadmium cell.
The hydrogen-absorbing alloys which have been developed so far have an ability of absorbing and desorbing a large amount of hydrogen, namely, has a high energy density, in the room temperature, where a cell is usually used. However, these alloys result in low initial charge/discharge efficiency and thus in insufficient electrochemical capacity on the initial stage of charge/discharge cycles. More specifically, an electrode using such a hydrogen-absorbing alloy is easy to charge from the initial stage, but the process of diffusing hydrogen from the inside to the outside of the alloy is a rate-determining step and so the polarity is changed with hydrogen remaining in the alloy. This phenomenon deteriorates the discharge efficiency.